1. Field of the Invention
The present invention relates to a piezoelectric ceramic and a piezoelectric device applicable to, for example, a piezoelectric vibrator such as an actuator, a piezoelectric buzzer, a sound component and a sensor, and specifically suitable for a multilayer piezoelectric vibrator.
2. Description of the Related Art
One of conventional devices using displacement generated by a piezoelectric effect as a mechanical driving source is an actuator. Specifically, compared to an electromagnetic actuator, a multilayer actuator in which a piezoelectric layer and an internal electrode are laminated has lower power consumption, a lower heating value and better responsivity, and the size and the weight of the multilayer actuator can be reduced. Therefore, in recent years, the multilayer actuator is used in various fields such as a needle selection control of a fabric-knitting machine.
Piezoelectric ceramics used for these actuators are required to have large piezoelectric properties, specifically a large piezoelectric constant d. As a piezoelectric ceramic capable of obtaining a large piezoelectric constant d, for example, a ternary system of lead titanate (PbTiO3; PT), lead zirconate (PbZrO3; PZ) and lead zinc niobate (Pb(Zn1/3Nb2/3)O3) (for example, refer to Japanese Examined Patent Application Publication No. Sho 44-17344 and Japanese Unexamined Patent Application Publication No. 2001-181035) and the ternary system to which bismuth (Bi) and zinc (Zn) are added (for example, refer to Japanese Unexamined Patent Application Publication No. Hei 3-40965) are known. Further, the ternary system in which strontium (Sr), barium (Ba), calcium (Ca) or the like is substituted for a part of lead (Pb) has been proposed (for example, refer to Japanese Examined Patent Application Publication No. Sho 45-39977 and Japanese Unexamined Patent Application Publication Nos. Sho 61-129888 and 2001-181036).
However, in a conventional piezoelectric ceramic, the firing temperature is as high as approximtely 1200° C., so when a multilayer piezoelectric device is manufactured, it is necessary to use an expensive noble metal such as platinum (Pt) or palladium (Pd) for the internal electrode, which causes a high manufacturing cost. Therefore, in order to use a lower-cost material for the internal electrode, a reduction in the firing temperature is desired.
For example, in the case where a lower-cost silver-palladium (Ag—Pd) alloy is used for the internal electrode, a oxidation-reduction reaction on palladium occurs during firing, which may cause a crack or peeling, so the palladium content is required to be 30 mass % or less, and for that purpose, according to the phase diagram of the silver-palladium system, the firing temperature is required to be 1150° C. or less, preferably 1120° C. or less. Moreover, in order to further reduce the manufacturing cost, it is preferable to further reduce the palladium content. For example, in order to contain the 20 mass % or less of palladium, the firing temperature is required to be 1050° C. or less, and in order to contain 15% or less of palladium, the firing temperature is required to be 1000° C. or less.
Moreover, using a lower-cost copper (Cu) for the internal electrode has been considered recently. However, the melting point of copper is 1085° C., so the firing temperature is required to be 1050° C. or less. In addition, copper is a base metal, so copper is oxidized when copper is fired in an air atmosphere, thereby copper cannot be used as the electrode. Therefore, it is necessary to fire copper in a low oxygen reducing atmosphere.
Moreover, in general, a proportional relationship is established between the piezoelectric constant d, an electromechanical coupling factor kr and a relative dielectric constant ∈r, so in the conventional piezoelectric ceramic, in order to obtain a large piezoelectric constant d, the electromechanical coupling factor kr and the relative dielectric constant ∈r are increased. However, when the relative dielectric constant ∈r increases, the impedance declines, so when the piezoelectric device is driven, a large amount of current flows through the piezoelectric device, thereby an energy required for driving the piezoelectric device is increased.
In addition, recently, the piezoelectric actuator is used in an automobile engine, a hot-melt type ink jet printer or the like, so the operating temperature may reach approximately 150° C. Therefore, a temperature eliminating piezoelectric properties, that is, a so-called Curie temperature is desired to be 300° C. or more.